Now that the threat of freezing drizzle (frizzle?) has passed, forecasters say up will be down and down will be up, as a strong temperature inversion and high pressure settle over Northwest Oregon and Southwest Washington.

Think of an inversion as a blanket of warm air that traps cold air beneath it, says Beth Burgess, a forecaster for the National Weather Service in Portland.

“That blanket traps cold air and pollutants at the surface and doesn’t allow the air to rise like it would normally,’’ Burgess said. “It just can’t happen…”

Forecasters were able to watch the inversion sink over Northwest Oregon early Tuesday morning via the dramatic temperature swing at Timberline Lodge. At 2 a.m. it was 24 degrees at the lodge; by 3 a.m., it was 51 degrees--at 27 degree shift in less than an hour.

This extended period of cool, dry weather will allow temperatures to drop to near freezing or below at least through next Monday, with a good chance for freezing fog.

View full sizeScenes like this will be more common for the next week or so, as a temperature inversion keeps cold air trapped at the surface, allowing temperatures near Mount Hood to soar into the 50s. This shot was taken last November from Oregon Health Sciences University near the Portland Tram.Brent Wojahn/The Oregonian

Daytime temperatures will moderate a bit as the week progresses, with highs reaching toward 50 degrees by Friday. A weak, winter sun will have a hard time burning off that fog, but there still could be afternoon clearing, Burgess said.

The inversion will also trap pollutants in the lower atmosphere. With colder temperatures, wood smoke, vehicle exhaust and other exhaust gases can quickly turn the air we breathe to an unhealthy soup.

To that end, the Oregon Dept. of Environmental Quality issued an air stagnation advisory for parts of south-central, southwest and north-central Oregon through Friday. That advisory could expand, officials said.

DEQ said the inversion has already made air quality in the affected areas worse than it was during last August's forest fires.

“We can hope that we see some mixing of the atmosphere and we’re not stuck with a 300-foot cloud ceiling and a protracted inversion,” Burgess said. “But right now it doesn’t look like it will happen anytime soon. Most of the time, it takes cold air to clear out an inversion.”